Preparation of chlorine by electrolysis of hydrochloric acid and polyvalent metal chlorides

ABSTRACT

The present invention relates to a dual electrolyte system utilizing a diaphragm electrolytic cell. The anolyte and catholyte, containing aqueous HCl and a polyvalent reducible metal chloride, are processed and recycled separately. The system of the present invention produces high-purity C12 at higher than conventional current efficiencies.

I United States Patent [151 3,635,804 Gritzner et al. [451 Jan. 18, 1972 4] PREPARATION OF CHLORINE BY' [56] References Cited ELEC'TROLYSIS OF HYDROCHLORIC UNlTED STATES PATENTS ACID AND POLYVALENT METAL CHLORIDES 3,486,994 12/ 1969 Donges et a1. ..204/128 2,468,766 5/1949 Low ..204/l28 [72] Inventors: Gerhard Grllzner; James J. Leddy, both of Midland, Mi h. Primary Examiner-J. H. Mack Assistant Examiner-R. L. Andrews [73] Asslgneez The Dow Chemlcal Comm! Midland Atmmey-Griswo1d & Burdick, Stephen S. Grace and William R. Norris [22] Filed: July 24, 1969 [57] ABSTRACT [21] Appl. No.: 844,402

The present invention relates to a dual electrolyte system utilizing a diaphragm electrolytic cell. The anolyte and [52] US. Cl ..204/l28 catholyte, containing aqueous HQ] and a polyvalent reducible Int-CL metal chloride, are processed and recycled separately. The [58] Field of Search ..204/ 128 system f the present invention produces high purity 1 at higher than conventional current efficiencies.

' 5 Claims, 1 Drawing Figure 6/ 6/3 Dryer Condenser 1 Como @0567 9605,25

Pa wer HC/ Con fem/n9 6/ 905 ource cona ensofe Reduced 15 a ,z fxlf oses H6 /-c on/a/n/ng 5 (0/7 ensa e I: '2 32 19 16 Dcp/e/eo;

0/70/y/? g C 1 0 O 12 5; f 11 X/ 36/ //)/750rz5@r 21 4/ 21 I flno/yi ggc/s CO/hO/yL gyg/g Oxygen -C0n7 0/ /7 m source Z 0 A s rel-0 677/5 8 2 Aeoxid/ e0 HC/ 616/ 5 ca/hofi/ fe PATENTEU man a 147' TORNE Y PREPARATION OF CHLORINE BY ELECTROLYSIS OF HYDROCHLORIC ACID AND POLYVALENT METAL CIILORIDES BACKGROUND of the INVENTION One method of producing Cl by electrolysis of HCl utilizes a polyvalent metal chloride in the electrolyte solution. For example, U.S. Pat. No. 2,468,766 describes an electrolysis method which comprises: introducing an electrolyte containing HCl and a polyvalent metal chloride, e.g., CuCl,, into the space between anode and cathode of a nondiaphragm cell, liberating Cl at the anode, reducing the polyvalent metal chloride at the cathode, withdrawing the electrolyte through the porous cathode and reoxidizing the polyvalent metal chloride with air and I-ICl for recycle. Such a method does reduce the cell voltage normally necessary for direct electrolysis of I-ICl. However, current efficiency suffers due to the inherent flow characteristics of the system which permit back reaction of dissolved chlorine and the lower valence state metal chloride.

German Pat. No. 1,277,216 discloses an electrolysis system which attempts to change the electrolyte flow pattern by use of a diaphragm between the anode and cathode. By maintaining a pressure differential of zero, the diaphragm reduces reaction of the dissolved Cl (anode side) with the lower valence metal chloride (cathode side). Although this does increase the current efficiency, there is still room for improvement. The German system carries out the reoxidation of the polyvalent metal chloride inside the cathode compartment of the cell. Since reoxidation is the slowest reaction taking place in the cell, this limits cell efficiency and cell size. Furthermore, improved results are only obtained using oxygen as the oxidizing gas.

It is a principal object of the present invention to provide a method and system of preparing Cl by electrolysis of HCl and a polyvalent metal chloride.

A further object of the present invention is to provide such a method and system which has high current efficiency.

THE INVENTION The above and other objects and advantages are found in the present invention which utilizes separate, dual-stream flow of electrolyte containing hydrochloric acid and a reducible polyvalent metal chloride. The invention employs an elec trolytic cell with a diaphragm which divides the cell into into anode and cathode compartments. Anolyte and catholyte are fed into the anode and cathode compartments. Preferably the cell is operated to achieve essentially zero fluid flow through the diaphragm.

The anolyte passes through the anode compartment where the hydrochloric acid is electrolyzed to form gaseous G The Ch-containing anolyte is withdrawn from the cell and the gaseous chlorine separated from the remainder of the anolyte. The anolyte can be replenished in hydrochloric acid by absorption of I-ICI and recycled through the anode compartment.

The catholyte passes through the cathode compartment where polyvalent metal chloride is reduced to the lower valence state metal chloride, e.g., CuCl, reduced to Cu Cl This reduced catholyte is removed from the cathode compartment. The metal chloride can be reoxidized to the higher valence state and the reoxidized catholyte recycled through the cathode compartment.

The dual-stream method of the present invention produces high-purity chlorine at extremely high current efiiciency, on the order of 95 percent, indicating a minimum of back reaction in the cell between the chlorine produced and the lower valence state metal chloride. Further the reoxidation of the catholyte takes place outside of the cell which permits use of more versatile equipment and apparatus. This permits more efficient operation of the system since the reoxidation rate does not limit the cell reaction.

The anolyte and catholyte can contain hydrochloric acid and any polyvalent metal chloride which exists in at least two oxidation states, e.g., copper chloride, iron chloride and chromium chloride. Copper chloride is preferred. Although concentration ranges can vary quite widely and the concentra tions can differ between catholyte and anolyte, a particular preferred concentration range is from about 0.5 to about 2 molar CuCl and from about 3 to about 8 molar I-ICl. In fact, if diaphragm and process conditions are appropriate, the catholyte can be essentially polyvalent metal chloride and/or the anolyte can be essentially HCl.

The reactions taking place in the system of the present invention using an HCl-CuCl, electrolyte are as follows:

Cathode reaction: 2CuCl Cu Cl,+2Cl' Anode reaction: 2HCI ZI-IHCI,

Oxidation reaction: Cu Cl,+%O,+2H +2Cl 2CuCl,+H,O These reactions add up to an overall reaction of PREFERRED EMBODIMENTS The FIGURE is a schematic flow diagram illustrating one embodiment of the dual flow electrolysis system of the present invention.

Referring to the FIGURE, electrolytic cell 10, composed of cathode ll, anode l2 and diaphragm 13, is connected to power source 14. The materials of construction of the electrodes are those normally employed in electrolytic cells, such as carbon or graphite. Diaphragm materials can include synthetic materials which retain substantial strength and dimensional stability such as polypropylene and copolymers of vinyl chloride and acrylonitrile, and polyvinyl chloride.

The anolyte cycle comprises feeding anolyte containing hydrochloric acid and a polyvalent metal chloride is fed into the anode compartment A of the cell. The HCl in the anolyte is electrolyzed to produce Cl gas. The anolyte is removed from the cell and the chlorine gas separated in a degassing unit 15. The depleted anolyte passes through heat exchanger 22 into an absorption tower 16 where it is contacted with HCl in gaseous or liquid form, preferably gaseous. HCl is absorbed in sufficient amount to replenish the HCl consumed during electrolysis. The replenished anolyte is then recycled through a filter 23 into the cell.

The Cl gas produced by the present method can be further processed by passing the gas through a condenser 17, which removes a substantial portion of the HCl in the Cl: gas, and drying in chlorine dryer l8, e.g., utilizing H The HClcontaining condensate can be added back to the anolyte.

In the catholyte cycle, the catholyte is fed through the cathode compartment C where polyvalent metal chloride is reduced to the lower oxidation state. The reduced catholyte is withdrawn from the cell and carried through heat exchanger 22 to an oxidation tower where it is oxidized, for example by admixing it ,with an oxidizing atmosphere, e.g., oxygen-containing gas or dilute chlorine-containing gas, to reoxidize the metal chloride. While the oxidizer I9 is shown in the FIGURE as one utilizing concurrent flow, it is understood that any tower or tank unit or the like which permits contact of the oxidizing atmosphere with the reduced anolyte is within the scope of the present system. The reoxidized anolyte is then recycled through a filter 23 into the cathode compartment.

The exit gases blown out of the oxidizing tower contain some l-ICl which can be recovered by passing the gases through a condenser 20. The I-ICl-containing condensate can be recycled to the oxidizer. A more elaborate recovery system, which can be used but is not necessary to the present invention, is described in U.S. Pat. No 2,666,024.

The anolyte and catholyte can be cycled for example by means of pumps 21 or other conventional means. It is preferable to control temperature which can be done by use of heat exchangers 22. It is also desirable in many instances to filter the incoming electrolyte streams to remove any solids. Any type of filter means 23 can be employed.

While the FIGURE shows a single cell system, it is understood that a series of cells connected in series could be employed in the present invention. For example, the cells could be arranged so that one side acts as the cathode for one cell and the other side acts as an anode for a second cell.

The following examples serve to further illustrate the present invention. The following general procedure and apparatus were used for the experiments. Unless otherwise indicated the term electrolyte refers to anolyte and catholyte.

A dual-flow system similar to the FIGURE was set up. Catholyte and anolyte were circulated separately by means of pumps. The flow rate could be regulated from to 650 ml./min. The flow of anolyte and catholyte was measured by means of two rotameters. Copper (1) chloride formed on the cathode was reoxidized with oxygen or air in the oxidizer. The latter was made out of Pyrex Glass, having an active volume of about 66 in. (21 inches high). The exit gas stream passed through a condenser. Oxygen or air was fed into the oxidizer through a sintered glass disc (medium size). The absorption of hydrochloric acid took place in a glass tube. The tubing for the connection was either polyethylene or pure gum rubber.

The electrode used for the cell was machined from a graphite block. The active electrode area was in.'*, and the electrodes were spaced one-sixteenth from the diaphragm. All other parts except the electrode area itself were painted with Saran-type cement in order to prevent electrolysis on those surfaces. Teflonor Saran-type nipples were used for the inlets and outlets of the electrolyte. Two of these electrodes were clamped together with a cloth diaphragm between them to form the electrolytic cell. Leaks were minimized by a Silastic film along the 1-inch frame of the cell by painting the same area on the diaphragm with Saran cement and by melting the edges of the diaphragm. A copper plate to which the copper leads were soldered was bolted to the external side of each graphite plate by means of two short brass bolts. The complete setup was placed in a temperature-controlled hotbox.

Hydrochloric acid concentration was determined by titration with l N NaOH sol. using methyl red as the indicator.

Copper (11) concentration was measured in the catholyte by the standard idometric way.

Copper (1) concentration was determined by the following procedure. A sample was quickly added to a ferric ammonium sulfate solution in sulfuric acid. The amount of iron (11) formed was oxidized to iron (ill) with 0.1 N ceric sulfate solution using Ferroin as indicator.

Chlorine was absorbed in 600 ml. of a Kl solution (400 g./l.) in sulfuric acid, diluted to 1,000 ml., and aliquots were titrated with 0.1 N sodium thiosulfate.

Gas analyses were made with the Orsat Method.

Current efficiency is determined by comparing the actual amount of C1 produced with that theoretically producable.

TABLE 1 Conditions:

Composition of electrolyte: 1.505 M CllClz, 5.8 M HCl. Electrolyte flow: 610 ml./min. Diaphragm: Copolymer of vinyl chloride and acrylonltrile. Regiostgrie between anode and cathode connectors: 33 milllohms Table I shows the current efficiencies of the present system at various current densities and corresponding voltage at two temperatures. As indicated, extremely high current efficiencies, 95 percent or above in almost all cases, are achieved even at very low current densities, i.e., 0.33 a./in. and correspondingly low cell voltages, i.e., less than 1 volt.

TABLE 11 Conditions:

Composition of electrolyte: 1.46 M CuCh, 5.03 M 1101. Electrolyte flow: See table below.

Diaphragm: Polypropylene.

Resistance: 17 milllohms.

Temperature: 7070.5 C.

Current Electrolyte Current density, flow rate, etllelency, amps/in. Volts mlJmin. percent Example Ne 11 0. 5 0. 890 010 95. 97 l). 5 0. 805 510 95. 07 0.5 0. 895 400 05. 07 0. 5 0. 000 300 05. 32 0. 5 0. 910 213 115. 75 1.00 1. 055 610 97. on 1.00 1. 07 510 97. en 1. 00 1. 10 400 07.61! 1.00 1.17 300 .17. 47 0. 33 0. 825 510 J2. 75 0.50 0.800 510 95. 32 0. 67 0. 045 510 06. 13 0. 83 0. 995 510 .17. 13 1. 00 1. 060 510 07. 68

Table II shows two aspects of the present system. First, high current efficiencies can be achieved over a wide range of electrolyte flow rates (examples 11-15 and l6l9). These examples showed little if any decrease in current efiiciency when the flow rate is decreased. Second, current efficiency can be increased by increasin g the current densitylexamples 20 24).

TABLE III Conditions:

Composition of electrolyte: 1.40 M 01101 0.0 M HCl. Electrolyte flow: 510 ml.lmin.

Die hragm: Polypropylene. Res stance: 17 mllliohms. Temperature: 7070.5 C.

Amt. Cu Current 0 flow in in reox. Current density, oxidizer, catholyte, efiic amps/1n. Volts ml./mln. g./1. percent Example No.:

the ability to produce Cl, at relatively high efficiencies even with incomplete reoxidation of the polyvalent metal chloride.

Cl 99.69% 00 -024 0, 0.01 N, oes

Thus the system and method of the present invention can be employed to produce high-purity C1 from electrolysis of HCl and a polyvalent metal chloride at very high current efficiencies over wide ranges of electrolyte concentrations, current densities and flow rates.

EXAMPLES 37-41 A larger diaphragm cell, having an active electrode area of 64 square inches, was incorporated into the system of the present invention. The cell design was similar to that used in the previous examples. Air at a flow rate of about 42.5 liters/min. was used in the oxidation tower as the oxidizing gas.

Table 111 reflects yet another feature of the present system Diaphragm: Polypropylene. Resistance: Not measured.

1 Active electrode area of 56 sq. in; diaphragm to electrode distance of 7 glIlC1L V7 Table IV demonstrates the high current efficiencies achieved by the present system and method using air as the oxidizing gas over a wide range of electrolyte flow rates. Examples 40 and 41 were taken from a continuously operated miniplant setup.

EXAMPLES 42-43 if desired, the present system can be operated with different HCl concentrau'ons in the anolyte and catholyte, with the same excellent current efficiencies, as shown by the following examples using the cell of examples 404 1 TABLE V Composition of electrolyte: 1.50 M CIIClg, 5.5 M H01. Electrolyte flow: 180 mL/min.

Diaphragm: Polypropylene.

Resistance: Not measured.

Temperature: 7273 C.

HCl cone, moles/ Current liter Current deusit e amps/in Volts Anolyte Catholyte percent asrc sthssiiae t s t What is claimed is:

l. A continuous process for preparing chlorine which comprises:

a. passing separate anolyte and catholyte streams through anode and cathode compartments respectively of an electrolysis cell divided into said compartments by a diaphragm; said anolyte and catholyte containing aqueous HCl and a polyvalent metal chloride; whereby Cl gas is produced at the anode and polyvalent metal chloride is reduced from a higher valence state to a lower valence state at the cathode; said cell being operated at a current density of up to about 1 ampere per square inch;

b. removing the Cl gas-containing anolyte stream from the anode compartment and the reduced catholyte stream from the cathode compartment;

c. separating the Cl gas from the anolyte stream;

d. feeding HCl into the anolyte stream to replenish the HCl consumed in the electrolysis and recycling the replenished anolyte stream into the cell anode compartment;

e. feeding an oxidizing atmosphere into the reduced catholyte stream to reoxidize polyvalent metal chloride and recycling the reoxidized catholyte stream into the cell cathode compartment, thereby to achieve a current efficiency of at least about percent when the polyvalent metal chloride is completely reoitidi z ed.

2. The process of claim 1 including the additional step of operating the cell so that essentially no fluid flow occurs 3. The process of claim 1 wherein the polyvalent metal ide s eop ersh ar e 4. The process of claim 1 wherein the compositions of catholyte and anolyte comprise from about 0.5 to about 2 M 92gb and from about 3 to about 8 M HCI v v 5. The process of claim 1 wherein the oxidizing atmosphere"" is air. 

2. The process of claim 1 including the additional step of operating the cell so that essentially no fluid flow occurs across the diaphragm.
 3. The process of claim 1 wherein the polyvalent metal chloride is copper chloride.
 4. The process of claim 1 wherein the compositions of catholyte and anolyte comprise from about 0.5 to about 2 M CuCl2 and from about 3 to about 8 M HC1.
 5. The process of claim 1 wherein the oxidizing atmosphere is air. 